The short (and relatively uninformative answer) is whatever you are comfortable with and trained to use. You will do quite well with Trusbx's list for a basic beginning. One thing I would definitely include would be backboards, both long and short, because you are likely to encounter trauma. Don't bother with anything commercial - get thee hence to the local lumber yard and make your own - instructions are easy to find, and you can customize it for your particular circumstances.

One thing about your post puzzles me. You are asking about a car kit for use in "wilderness terrain." For me, wilderness is by definition roadless, so I imagine you are talking about driving in wild country with rough roads, but not wilderness, strictly speaking, as I understand the term. This is significant, because you are likely to be able to transport toward medical help.

Probably you should consider a means of communication as part of your medical support system. A helicopter ride is more likely to be life saving than a suture kit in most circumstances, as is the ability to provide a warm snug environment for the victim to prevent shock while awaiting transport.